Discovery of a damped Ly α absorber in the circumnuclear zone of the FeLoBAL quasar SDSS J083942.11+380526.3

Abstract

ABSTRACT SDSS J083942.11+380526.3 ($z=2.315$) is an iron low-ionization broad absorption lines quasar that exhibits visible Balmer absorption lines (H $\alpha$), implying a significant $n=2$ population. The quasar also shows an array of absorption lines, including O i, Ni ii, Fe ii, Mg ii, Al iii to C iv, and N v. The high-ionization absorption lines such as C iv and Si iv are revealed by slightly blueshifted broad absorption lines troughs. The resonance doublets such as Mg ii and Al iii are saturated but did not reached zero intensity which indicates that the broad-line region (BLR) is partially covered. Overall, however, the absorption is predominantly from low-ionization Fe ii lines, emitted from ground and excited states up to at least 3.814 eV. This implies that the absorbing gas spans the hydrogen ionization front and extends into the partially ionized zone where neutral hydrogen is certainly present. Notably, the hydrogen line spectrum of the quasar shows no signature of expected Ly $\alpha$ absorption. Instead, the line spectrum shows an unusual Ly $\alpha$ emission characterized by a fully filled emission line spectrum which is a composite of a strong narrow core superposed on a weak broad base. Taking into account the effect of partial covering to BLR, we have extracted a strong Damped Ly α systems (DLA) trough in Ly $\alpha$ emission region. To fit the spectrum, we performed photoionized model calculations and compared them to the observations. We found that photoionization modelling using cloudy can successfully reproduce the main characteristics of the quasar spectrum, and the predicted neutral hydrogen column density arising from the clouds responsible for the low-ionization absorption provides a good match to the extracted DLA trough. This indicates that both the DLA and the low-ionization absorption arise from the same medium that is roughly collocated with the dusty torus.